Method for scanning screened image master images

ABSTRACT

A method for opto-electronic scanning, by pixel and by line, of screened master photographic images with a sensing device. From the actual scanning, at least one master photographic image area that contains the grid scale is detected in the screened master photographic image. Subsequently, the picture values present in the local area of the detected grid scale area of the master photographic image are transferred by Fourier transformation to the frequency range as a spatial frequency spectrum, and, from the spatial frequency spectrum, the screen ruling and raster angle of the raster of the master photographic image are determined. For the determined raster angle and the determined screen rulings of the grid scale of the master photographic image, the optimal scan frequencies for Moiré-free scanning are determined from the spatial frequency spectrum. The optimal scan frequencies which determined the scanning fineness are set at the sensing device and then the screened master photographic image is scanned with the set optical scan frequencies, to obtain the picture values necessary for reproduction of the screened master photographic image.

BACKGROUND OF THE INVENTION

The invention is in the field of electronic reproduction technology andis directed to a method for pixel-by-pixel and line-by-lineopto-electronic scanning of screened image masters.

In electronic reproduction technology, image values are acquired andfurther-processed in a scanner device, also referred to as a scanner, bypixel-by-pixel and line-by-line scanning of an image master to bereproduced with an opto-electronic scanner element. The scanner elementis essentially composed of a light source for the illumination of theimage master to be scanned and of an opto-electronic transducer. Thescanner can be designed as a flat bed scanner or as a drum scanner.

In practice, screened image masters must often be scanned. The raster ofan image master to be scanned is defined by the screen frequency (screendots/cm) and by the screen angle, whereas the scan fineness in thescanning of the image master is determined by the scan frequency(pixels/cm), whereby screen frequency and scanning frequency are spatialfrequencies.

Beat frequencies as sums and differences between the sampling frequencyand the harmonics of the screen frequency arise when scanning a screenedimage master. Low beat frequencies cause a visible Moiré that a viewerconsiders disturbing and, thus, diminishes the reproduction quality.

Patent Abstracts of Japan, Vol. 096, No. 006, Jun. 28, 1996, and JP 08051536 A already disclose a method for avoiding moiré in scanningmasters that arises due to image enlargement. In this known method, theimage values are subjected to a Fourier transformation for therecognition of moiré, and the moiré is eliminated by a following spatialfiltering.

Patent Abstracts of Japan, Vol. 097, No. 001, Jan. 31, 1997 and JP 08242364 A disclose another method for avoiding moiré when scanningmasters on the basis of a spatial filtering.

EP 0 511 754 A and EP 0 301 786 A recite methods for the descreening ofscreened masters by filtering.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to improve a methodfor scanning screened image masters such that disturbing Moiré phenomenaare avoided or are at least greatly reduced.

According to the present invention, a method is provided forpixel-by-pixel and line-by-line opto-electronic scanning of a screenimage master with a scanner device. Before the actual scanning of themaster, at least one master area as a screen area containing a screen isidentified in the screened image master. Image values of the identifiedscreen area of the image master present in a locus domain are convertedby a Fourier transformation into a frequency domain as a spatialfrequency spectrum. Screen width and screen angle of the screen of theimage raster are identified from the spatial frequency spectrum. Scanfrequencies from the spatial frequency spectrum that are optimum for aMoiré-free scanning are determined for the identified screen angle andthe identified screen width of the screen of the image master. Theoptimum scan frequencies that determine the scan fineness are set at thescanner device. The screen image master is scanned with the set optimumscan frequencies for acquiring image values required for reproduction ofthe screened image master.

The invention is explained in greater detail with reference to FIGS. 1and 2.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a frequency spectrum; and

FIG. 2 illustrates an enlarged portion from the frequency spectrum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventive method for pixel-by-pixel and line-by-line opto-electronicscanning of screened image masters is described in greater detail below.The image masters are, for example, screened black/white masters,screened color separations or printed color images that contain no grayscale values (contone). The screened image masters can contain areaswith text and areas with screened images (screen areas).

In a first method step, at least one “screen area” containing the screenstructure is identified in the screened image master by evaluating theimage value.

For that purpose, the screened image master is first scanned in ascanner with a coarser scanning fineness then the scanning finenessrequired for the reproduction of the image master (coarse scan). In thecoarse scan, a low-pass filtering of the scanned image information issimultaneously implemented. What the low-pass filtering achieves is thatgray scale values are generated despite scanning the raster points asblack/white information in the image master. The low-pass filteringoccurs, for example, on the basis of an unsharp scanning with amagnified picture element diaphragm.

The functioning of a scanner is notoriously known, so that a moredetailed description of its functioning is superfluous. For example, theflat bed scanner “TOPAZ” of Linotype-Hell AG can be employed.

When the scanner is calibrated such that the image value is 100% whenscanning “white” and 0% when scanning “black”, an area of the master isthen recognized as a “screen area” when the scanned image values of thecoarse scan are gray scale values in the medium tone area of, forexample, 30% through 70%. At the same time, the boundaries of the screenarea are marked by locus coordinates.

The coarse scan of the screened image master can be aborted as soon as a“screen area” has been found in the scanned image master, as a resultwhereof scan time in the coarse scan is advantageously saved.

In a second method step, the identified “screen area” of the imagemaster is scanned in the scanner with a higher scan fineness (fine scan)compared to the scan fineness of the coarse scan, and the image valuesof the “screen area”, for example 512×512 image values, are stored forlater evaluation. The limitation of the scan to the “screen area” of theimage master occurs with the assistance of the previously identifiedlocus coordinates.

In a third method step, the image values of the “screen area” that arepresent in the locus domain are converted into the frequency domain by atwo-dimensional Fourier transformation FT, preferably by a fast Fouriertransformation (FFT). Such Fourier transformations are likewise knownand described, for example, in Rabiner, L. R., “Theory And ApplicationOf Digital Signal Processing”, Chapter 6, 1975 ISBN 0-13-914101-4.

FIG. 1 shows the spatial frequency spectrum of the screen of the scannedimage master for a screen angle of, for example, 15°. When scanningscreened color separations with different screen angles, a correspondingspatial frequency spectrum derives for each screen angle. The maximumsof the spatial frequencies (harmonics) of the screen frequency f_(R)that are defined by the frequencies f_(x) f_(y) lie in the intersectionsof the lines of the grid network 1. The frequency f_(x) is allocated,for example, to the line direction and the frequency f_(y) is allocatedto the feed direction during the scanning of the master in the scanner.

A sampling frequency f_(Ax) and a sampling frequency f_(Ay) are enteredon the frequency axes of the spatial frequency spectrum. Given pixelsassumed to be square, the image master being resolved into these duringthe scanning, the scanning frequencies in the direction of the f_(x)frequency axis and of the f_(y) frequency axis of the spatial frequencyspectrum are equal f_(Ax)=f_(Ay), so that only a quadrant or,respectively, a frequency axis of the spatial frequency spectrum can beobserved later.

In a fourth method step, screen angle and screen width of the screen ofthe screened image master are identified from the spatial frequencyspectrum according to defined search criteria. This can occurs, forexample, by recognizing the maximums in the spatial frequency spectrumin that respectively first harmonics are investigated. The determinationof screen angle and screen width can preferably occur with an automaticanalysis of the spatial frequency spectrum in a scanner or in a workstation.

In a fifth method step, suitable scanned frequencies f_(Ax)=f_(Ay) forthe actual scanning of the screened image master are sought from thespatial frequency spectrum for the screen angle and the screen widththat had been identified in the fourth method step.

For the purpose, FIG. 2 shows an enlarged portion from the spatialfrequency spectrum of FIG. 1 along the f_(x) frequency axis. Suitablesampling frequencies f_(Ax) can lie in each of the grid meshes 2 of thespatial frequency spectrum on the f_(x) frequency axis. The suitablescanning frequency f_(Ax) in a grid mesh 2 is identified by a distanceevaluation of the corresponding harmonic frequencies (corner points ofthe grid mesh). For example, that frequency that has the same spacingfrom the two closest harmonic frequencies of the corresponding grid mesh2 is determined as suitable scan frequency f_(Ax) in a grid mesh 2.

FIG. 2 shows the suitable scan frequencies f_(Ax1) and f_(Ax2) in twogrid meshes 2, whereby the scan frequency f_(Ax1) is preferred over thescan frequency f_(Ax2), since it has the greater distances from theharmonic frequencies and, thus, a minimum of disturbing Moiré isachieved. In particular, f_(A)>f_(R) should apply.

That scan frequency f_(Ax) which can be optimally realized in thescanner provided for the scanning of the screened image master is thenselected from among the suitable scan frequencies f_(Ax). Given a pixelresolution assumed to be quadratic, f_(Ax)=f_(Ay) then applies.

In a flat bed scanner, the scanner element comprises a photo diode line(CCD line) oriented in the line direction as a opto-electronictransducer and comprises a vario lens for line-by-line imaging of theimage master onto the photodiode line. The image master to be scanned isarranged on a master table that moves perpendicular to the linedirection (feed direction). Given the flat bed scanner, the vario lenswith which the image master is imaged line-by-line onto the photodiodeline of the scanner element is set in the x-direction according to theoptimum scan frequency f_(Ax), whereas the optimum scan frequency f_(Ay)in the y-direction determines the feed rate of the master table.

In a drum scanner, the image master to be scanned is clamped on ascanner drum that rotates in the line direction (circumferentialdirection). The scanner element with a photo-multiplier as theopto-electronic transducer moves past the scanner drum in the axialdirection (feed direction). Given the drum scanner, the samplingfrequency in the analog-to-digital conversion of the image signal isselected according to the optimum scan frequency f_(Az) in x-direction,whereas the optimum scan frequency f_(Ay) in the y-direction determinesthe feed rate of the scanner element along the scanner drum.

In a sixth method step, the screened image masters to be reproduced arethen scanned again in the scanner as a fine scan with the previouslyidentified, optimum scan frequencies f_(Ax), f_(Ay) or, respectively,scan finenesses in the line and the feed direction, and the Moiré-freeimage values that are thereby acquired are further-processed.

When the screened image masters are, for example, the four colorseparations of a color set, the individual color separations aresuccessively scanned with the scan frequencies determined as optimum forthe respective color separation as a fine scan, and the Moiré-free imagevalues thereby acquired are digitally de-screened by a low-passfiltering. The half-tone color separations (contone separations) arethen calculated from the de-screened image values. Alternatively, theMoiré-free image values can also be further-processed as a line workwithout a corresponding low-pass filtering.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that my wish is toinclude within the claims of the patent warranted hereon all suchchanges and modifications as reasonably come within my contribution tothe art.

I claim:
 1. Method for pixel-by-pixel and line-by-line opto-electronicscanning of screened image masters with a scanner device, characterizedin that, before the actual scanning of the master, at least one masterarea (screen area) containing the screen is identified in the screenedimage master; the image values of the identified screen area of theimage master present in the locus domain are converted by a Fouriertransformation into the frequency domain as spatial frequency spectrum;screen width and screen angle of the screen of the image master areidentified from the spatial frequency spectrum; the scan frequencies(f_(Ax), f_(Ay)) from the spatial frequency spectrum that are optimumfor a Moiré-free scanning are determined for the identified screen angleand the identified screen width of the screen of the image master; theoptimum scan frequencies (f_(Ax), f_(Ay)) that determine the scanfineness are set at the scanner device; and the screened image master isscanned with the set, optimum scan frequencies (f_(Ax), f_(Ay)) foracquiring the image values required for the reproduction of the screenedimage master.
 2. A method for pixel-by-pixel and line-by-lineopto-electronic scanning of a screened image master with a scannerdevice wherein before the actual scanning of the master, performing thesteps of: identifying at least one master area as a screen areacontaining a screen identified in the screened image master; convertingimage values of the identified screen area of the image master presentin a locus domain by a Fourier transformation into a frequency domain asa spatial frequency spectrum; identifying screen width and screen angleof the screen of the image master from the spatial frequency spectrum;determining scan frequencies from the spatial frequency spectrum thatare optimum for a Moiré-free scanning for the identified screen angleand the identified screen width of the screen of the image master;setting the optimum scan frequencies that determine scan fineness at thescanner device; and scanning the screened image master with the setoptimum scan frequencies for acquiring image values required forreproduction of the screened image master.
 3. A method foropto-electronic scanning of a screened image master with a scannerdevice wherein before the actual scanning of the master, performing thesteps of: identifying at least one master area as a screen areacontaining a screen identified in the screened image master; convertingimage values of the identified screen area of the image master at alocus by a Fourier transformation into a frequency domain as a spatialfrequency spectrum; identifying screen width and screen angle of thescreen of the image master from the spatial frequency spectrum;determining scan frequencies from the spatial frequency spectrum thatare substantially optimum for a substantially Moiré-free scanning forthe identified screen angle and the identified screen width of thescreen of the image master; setting the optimum scan frequencies thatdetermine scan fineness at the scanner device; and scanning the screenedimage master with the set optimum scan frequencies for acquiring imagevalues required for reproduction of the screened image master.